Myeloid cells originate from multipotent hematopoietic stem cells in the bone marrow and consist of granulocytes (neutrophils, eosinophils, basophils) and cells of monocyte/macrophage lineage including dendritic cells (DCs) and osteoclasts. These cells play a critical role in innate and adaptive immunity, inflammatory reactions, and bone remodeling. Transformed myeloid cells give rise to neoplasia, such as acute and chronic myeloid leukemia. Substantial gains in the understanding of myeloid cell development and leukemogenesis have been made over the past several decades through identification, isolation, and targeted manipulation of hematopoietic stem cells and progenitors (Rosmarin, A. G., et al., 2005, Exp. Hematol. 33:131-143. Rosenbauer, F. and Tenen, D. G. 2007, Nat. Rev. Immunol. 7:105-117.). The majority of these studies are based on mouse models because of the ease with which mouse cells can be manipulated and assayed for hematopoietic lineage commitment potential. In vitro differentiation studies using human bone marrow cells are hampered by the limited availability of bone marrow myeloid precursors and the complexity of genetic manipulation of bone marrow cells. While myeloid leukemia cell lines are frequently used to study differentiation of myeloid cells, these cells have a highly abnormal karyotype and often display functional differences from their normal myeloid counterparts (Rado, T. A., et al., 1987, Blood. 70:989-993. Khanna-Gupta, A., et al., 1994, Blood. 84:294-302. Baumann, M. A., et al., 1998, Stem Cells. 16:16-24. Fischkoff, S. A. 1988, Leuk. Res. 12:679-686.).
Human embryonic stem cells (hESCs) are pluripotent stem cells capable of indefinite self-renewal and differentiation toward all three germ layers (ectoderm, endoderm, and mesoderm) (Thomson, J. A., et al. 1998, Science. 282:1145-1147.). In vitro differentiation of hESCs has provided a unique opportunity to study early hematopoietic commitment and specification of different hematopoietic lineages (Keller, G., et al., 1999 Nat. Med. 5:151-152). In addition, establishing conditions for directed differentiation of hESCs toward a particular hematopoietic lineage has allowed the functional analysis of genes essential for lineage expansion and maturation without limitation in terms of cell numbers and heterogeneity of progenitors.
Recently, pluripotent stem cell lines have been obtained from human fibroblasts through insertion of certain genes critical for the maintenance of pluripotency of hESCs (Yu, J., et al. 2007, Science. 318:1917-1920. Takahashi, K., et al. 2007, Cell. 131:861-872. Park, I. H., et al. 2008, Nature. 451:141-146.). These so-called human induced pluripotent stem cells (hiPSCs) behave similarly to hESCs, i.e., they are capable of self-renewal and large-scale expansion and differentiation toward all three germ layers. The hope is that hiPSC lines generated from patients with various diseases could be used to obtain any type of progenitor or differentiated cell carrying a particular genetic trait at the cellular level, thus providing a unique opportunity to analyze disease pathogenesis in vitro.
The Slukvin lab has established a system for efficient hematopoietic differentiation of hESCs into hematopoietic cells through coculture with OP9 bone marrow stromal cells (Vodyanik, M. A., Bork, J. A., Thomson, J. A., Slukvin, I. I. 2005, Blood. 105:617-626) and characterized the two subpopulations of the most primitive multipotent hematopoietic cells to appear in OP9 cocultures of hESCs on the basis of their common expression of CD43 and differential expression of CD45. The lin−CD34+CD43+CD45− cells with broad lymphomyeloid differentiation potential appear first in coculture. Later, lin−CD34+CD43+CD45+ cells enriched in myeloid progenitors emerge (Vodyanik, M. A., Thomson, J. A., Slukvin, I. I. 2006, Blood. 108:2095-2105.). Recently the Slukvin lab demonstrated that a similar pattern of hematopoietic differentiation is observed when hiPSCs differentiate into blood cells in coculture with OP9 (Choi, K., et al. 2009, Stem Cells. 27:559-567.).